Washington -LRB- CNN -RRB- -- Congress delved Wednesday into the politically explosive issue of unmanned drone attacks , questioning the legality of operations increasingly used to combat al Qaeda and Taliban militants in countries such as Pakistan .

In the eight years of George W. Bush 's presidency , unmanned aircraft -- or drones -- attacked militant targets 45 times .

Since President Obama took office , the numbers have risen sharply : 51 last year and 29 so far this year .

Most attacks have targeted suspected militant hideouts in Pakistan . While the United States is the only country in the region known to have the ability to launch missiles from drones -- which are controlled remotely -- U.S. officials normally do not comment on suspected drone strikes .

Based on a CNN count , all of the 29 drone strikes this year have hit locations in North Waziristan and South Waziristan , along the 1,500-mile porous border that Pakistan shares with Afghanistan .

Several top U.S. law professors debated the legality of the attacks in a hearing before the House Subcommittee on National Security and Foreign Affairs , the second such hearing held by the subcommittee within the past two months .

`` The United States is committed to following international legal standards , '' said Rep. John Tierney , D-Massachusetts , the subcommittee 's chairman . `` Our interpretation of how these standards apply to the use of unmanned weapons systems will set an example for other nations to follow . ''

The four legal scholars invited to testify , however , offered sharply contrasting views of what constitutes an acceptable legal standard . The biggest controversy appeared to surround the legality of strikes conducted by CIA operatives , as opposed to strikes by the U.S. military .

`` Only a combatant -- a lawful combatant -- may carry out the use of killing with combat drones , '' said Mary Ellen O'Connell , a professor from the University of Notre Dame law school .

`` The CIA and civilian contractors have no right to do so . They do not wear uniforms , and they are not in the chain of command . And most importantly , they are not trained in the law of armed conflict . ''

O'Connell also said that `` we know from empirical data ... that the use of major military force in counterterrorism operations has been counterproductive . '' The U.S. government , she said , should use force only `` when we can accomplish more good than harm , and that is not the case with the use of drones in places like Pakistan , Yemen and Somalia . ''

David Glazier , a professor from Loyola law school in Los Angeles , California , defended the drone attacks on the grounds that there is `` no dispute that we are in an armed conflict with al Qaeda and with the Taliban . '' That fact `` allows the United States to call upon the full scope of authority which is provided by the law of war . ''

Glazier said there is `` nothing within the law of war that prohibits the use of drones . In fact , the ability of the drones to engage in a higher level of precision and to discriminate more carefully between military and civilian targets than has existed in the past actually suggests that they 're preferable to many older weapons . ''

He conceded , however , that there are legitimate concerns about the CIA 's use of drones . CIA personnel are `` clearly not lawful combatants , -LSB- and -RSB- if you are not a privileged combatant , you simply do n't have immunity from domestic law for participating in hostilities . ''

Glazier warned that `` any CIA personnel who participate in this armed conflict run the risk of being prosecuted under the national laws of the places where -LSB- the combat actions -RSB- take place . '' CIA personnel , he said , could be guilty of war crimes .

William Banks , the founding director of Syracuse University 's Institute for National Security and Counterterrorism , said the U.S. government has engaged in targeted killings of individual combatants dating at least back to a 1916 border war with Mexican bandits .

Banks said the authors of the 1947 National Security Act , which traditionally gives the CIA much of its legal authority , probably did n't contemplate the targeted killings tied to drone attacks . But the statute , he said , was `` designed as dynamic authority to be shaped by practice and by necessity . ''

`` The intelligence laws permit the president broad discretion to utilize the nation 's intelligence agencies to carry out national security operations , implicitly including targeted killing , '' he said . U.S. laws `` supply adequate -- albeit not well-articulated or understood -- legal authority for these drone strikes . ''

The American Civil Liberties Union sent a public letter to Obama on Wednesday that said the drone attacks are part of an illegal program authorized by the administration allowing suspected terrorists -- including Americans -- to be targeted and killed by U.S. operatives .

`` The program you have reportedly endorsed is not simply illegal but also unwise , because how our country responds to the threat of terrorism will in large measure determine the rules that govern every nation 's conduct in similar contexts , '' ACLU Executive Director Anthony Romero said .

`` If the United States claims the authority to use lethal force against suspected enemies of the U.S. anywhere in the world -- using unmanned drones or other means -- then other countries will regard that conduct as justified . The prospect of foreign governments hunting and killing their enemies within our borders or those of our allies is abhorrent . ''

Peter Bergen , a fellow at the New America Foundation , a nonpartisan group , suggested that the increase in drone attacks during the Obama administration is , in part , revenge for the bombing of a CIA base in eastern Afghanistan that killed seven Americans on December 30 , 2009 .

`` The people who died in this suicide attack were involved in targeting people on the other side of the border , '' he said earlier this year .

Long War Journal , an online publication that charts data for U.S. airstrikes against al Qaeda and the Taliban in Pakistan , says the air campaign `` remains the cornerstone of the effort to root out and decapitate the senior leadership of al Qaeda , the Taliban , and other allied terror groups , and to disrupt both al Qaeda 's global and local operations in Afghanistan and Pakistan . ''

Such attacks , which have taken a civilian toll in many cases , have frequently caused tension between Pakistan and the United States .

CNN 's Alan Silverleib contributed to this report .

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NEW : ACLU calls drone attacks part of illegal program for U.S. to target , kill terror suspects

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Since President Obama took office , number of drone attacks has risen

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U.S. law professors debate legality of such attacks during a House subcommittee hearing

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Biggest controversy : legality of strikes conducted by CIA , as opposed to U.S. military